The rules don’t apply to Desmond, or Richard
I think Desmond has the same special ability as Richard. Perhaps time doesnt effect Desmond anymore, like Richard, so he wont age.
I’m not sure what the special ability Richard has not to age, but I feel Desmond has the same ability, as when Daniel says to him “The rules, the rules don’t apply to you”.
I dont think this means Desmond is as old as Richard, I think Desmond might have recieved this ability when he turned the hatch key.
I cannot prove this until we find out what Richard’s ability is, or if we see Demond in the future and he hasn’t aged.